There are various types of color display in practical use. Thin-shaped displays are classified into (i) a self-emission type display such as a PDP (plasma display panel) and (ii) a non-emission type display such as an LCD (liquid crystal display). A transmissive LCD is well known as the LCD, which is a non-emission type display. The transmissive LCD is configured such that a backlight is provided behind a liquid crystal panel.
FIG. 8 is a cross sectional view illustrating a general structure of such a transmissive LCD. In the transmissive LCD, a backlight 110 is provided behind a liquid crystal panel 100. The liquid crystal panel 100 includes a pair of transparent substrates 101 and 102, a liquid crystal layer 103 provided therebetween, and polarizing plates 104 and 105 sandwiching the transparent substrates 101 and 102. Further, a color filter 106 is provided in the liquid crystal panel 100, thereby attaining color display.
Although not illustrated, electrode layers and alignment films are provided on the inner sides of the transparent substrates 101 and 102. By controlling an applied voltage to the liquid crystal layer 103, a transmission amount of light through the liquid crystal panel 100 is controlled for each pixel. In other words, in the transmissive LCD, the liquid crystal panel 100 controls the transmission amount of light irradiated from the backlight 110, thus carrying out display control.
Mainly employed for the backlight 110 is a backlight emitting white light having respective wavelengths of R, G, and B, which are necessary for color display. The backlight 110 and the color filter 106 work together to adjust each of transmittances for the R, G, and B light beams of the light, thereby arbitrarily setting luminance and hue in each pixel. For the backlight 110, a backlight including light sources for R, G, and B can be used.
For example, in the LCD, transmittance for output display information is controlled by a shutter operation of the liquid crystal panel 100 provided with the color filter 106 having portions corresponding to R, G, and B existing in each pixel. Specifically, the transmittance is controlled in a range from 0% to 100% by performing predetermined steps. Consider a case of allowing light from the light backlight 110 to pass therethrough by 100%. In this case, ideally, the light irradiated from the backlight is outputted with intact intensities in its relevant color components, so luminance is the maximum. Meanwhile, when the transmittance is set to be 0%, black display is attained. As such, in such a normal transmissive LCD carrying out display control through the shutter operation of the liquid crystal panel 100, the backlight 110 keeps on emitting light with the constant luminance.
Because the backlight 110 thus keeps on emitting light with the constant luminance in the above structure, the backlight 110 consumes a lot of electric power. Specifically, even while the LCD displays dark images on its screen as a whole, the backlight 110 emits light with the maximum luminance. Most of the light thus emitted is blocked as a result of the shutter operation of the liquid crystal panel 100. As such, a large amount of light from the backlight 110 is wasted while electric power consumption in the backlight 110 is large. The electric power consumption of the backlight makes up a large proportion of electric power consumption in the LCD. Hence, such a waste is a very great loss for the entire system.
In order to solve the problem, Japanese Unexamined Patent Publication “Tokukai 2006-47594 (published on Feb. 16, 2006)” discloses a technique of reducing electric power consumption of backlights. In the technique, luminance adjustable active backlights are used to carry out display control of an LCD (luminance control) by controlling transmittance in the liquid crystal panel and luminance in the active backlight. FIG. 9 illustrates a schematic structure of the LCD system described in Japanese Unexamined Patent Publication “Tokukai 2006-47594 (published on Feb. 16, 2006)”.
The LCD shown in FIG. 9 is configured such that a CPU 202 sends image information, stored in a RAM 201, to an active BL (backlight) controller 203. The active BL controller 203 uses liquid crystal drivers 204 and 205 so as to control transmittance in the liquid crystal panel 210, and uses backlight luminance adjusting sections 206R, 206G, and 206B so as to control respective luminances of light from a red backlight 207R, a green backlight 207G, and a blue backlight 207B. Thus, the red backlight 207R, the green backlight 207G, and the blue backlight 207B in the LCD are active backlights each capable of adjusting the luminance of light therefrom.
With reference to FIG. 10(a) to FIG. 10(c), the following explains an effect of reducing electric power consumption of the backlights in Japanese Unexamined Patent Publication “Tokukai 2006-47594 (published on Feb. 16, 2006)”. For ease of explanation, the description below exemplifies display control over an area made up of 4 pixels, each of which has color components of R, G, and B.
First, consider a case of carrying out a display operation with 256 grayscale (0 to 255) in accordance with display data shown in FIG. 10(a). Here, while no luminance control is carried out with respect to the backlights, each of the backlights emits light with the maximum luminance (255), and only the transmittance of the liquid crystal panel is controlled in accordance with the display data (see FIG. 10(b)).
Meanwhile, consider a case of carrying out display control by using the active backlights as shown in FIG. 10(c). In this case, the luminance of light from the backlights is controlled to coincide with the maximum luminance value in the display data. In the case where the red, green, and blue backlights are provided as such, each of the red, green, and blue backlights controls the luminance of light therefrom such that the luminance thereof coincides with the maximum luminance value of a corresponding color component of the display data. In accordance with the luminance of each backlight, the transmittance of the liquid crystal panel is adjusted. For example, in FIG. 10(c), in order to display a red image at a display luminance of 128, the luminance of light from the red backlight is set at 128 and the transmittance of a relevant pixel in the liquid crystal panel is set at 255 (100%). In this way, the display luminance of 128 (=luminance of 128×transmittance of 100%) is attained. In the meanwhile, in FIG. 10(b), in order to display a red image at a display luminance of 128, the luminance of light from the red backlight is set at 255 and the transmittance of a relevant pixel in the liquid crystal panel is set at 128 (50%). Now, compare the cases of FIG. 10(c) and FIG. 10(b). The luminance of light from the backlight in Figure 10(c) is 128, which is reduced from the luminance of 255 in the case of FIG. 10(b).
In the transmissive display device using such an active backlight, the luminance of light from each active backlight can be controlled with the entire screen regarded as one area. However, by carrying out backlight luminance control for a plurality of divided areas of the screen individually, it is possible to enhance the effect of reducing electric power consumption.
However, such backlight luminance control for each of the divided areas of the display screen suffers from such a problem that boundaries between display areas are likely to be viewed and recognized due to light leaking from adjacent display areas. The following explains such a problem with reference to FIG. 11(a) to FIG. 11(c).
First, consider a case where a display operation is carried out in two adjacent display areas in accordance with display data shown in FIG. 11(a). For ease of explanation, assume that each of the two adjacent display areas is made up of three pixels in this case.
FIG. 11(b) illustrates the luminance of the backlight and the transmittance of each pixel, both of which are controlled in accordance with the display data shown in FIG. 11(a). In this case, light beams irradiated from light emitting areas of the backlight are not completely parallel to one another, so there is leaking light (illustrated by a solid arrow in FIG. 11(b)) going from one light emitting area to adjacent display areas. Such leaking light undesirably increases display luminances of pixels located in the vicinity of the boundary of the display areas as shown in FIG. 11(c). As a result, when the luminances in pixels adjacent to each other with a boundary of display areas therebetween should be the same, the actual display luminances therein become different from each other, with the result that a viewer is likely to view and recognize the boundary due to the difference of luminances.